The long-term objective of this research project is an in-depth investigation of the thermodynamic and structural properties of various chemically well-defined mixed-chain phospholipids in lipid bilayers. It has long been established that phospholipids are a major class of lipid components in the highly complex organization of biological membranes and that phospholipids isolated from biological membranes are typically of the mixed-chain variety, meaning that the two hydrocarbon chains are different in both the chemical structure and the physical strength. Our proposed studies of mixed-chain phospholipids may, therefore, have important structural and functional relevance in understanding the physical and biological nature of biological membranes. Specifically, the proposed five-year research is designed along the following lines: (1) Recently we have applied high-resolution differential scanning calorimetry (DSC), vibrational spectroscopy, 31P NMR, and X-ray diffraction to investigate the thermodynamic and structural properties of various mixed-chain phosphatidylcholines (PC) in the bilayer. We shall continue our investigation but concentrating on the most highly asymmetric PC. (2) Both the packing effect and the hydrophobic hydration may contribute to the unusually large transition enthalpy (deltaH) and entropy (deltaS) associated with the phase transition involving the mixed interdigitated bilayer. We shall determine which one of the two effects is more important. (3) We shall construct phase diagrams for binary phospholipid mixtures and to obtain quantitative information regarding the lipid-lipid interactions by simulating the shape of the phase diagram using a computer program developed in this laboratory. (4) The thermotropic phase behavior and structural properties of mixed-chain PE and their headgroup derivatives will be studied systematically. (5) Empirical predictions of the main phase transition temperatures (Tm) for mixed-chain phospholipids (PC, PE and derivatives) will be developed. (6) We shall examine the thermotropic phase behavior of two series of mixed-chain phosphatidylglycerol (PG) with molecular weights identical with C(16):C(16)PG and C(14):C(14)PG, respectively. The results of the research proposed in this project will add a new dimension to our current knowledge about the structure and properties of membrane lipids.